NOVELTY - Performing a shift reaction involves introducing a feed gas (1) comprising water (H2O) and carbon monoxide (CO) into a combo-boiler (14) for conversion of H2O and CO into carbon dioxide (CO2) and hydrogen (H2); introducing a first process gas into the tubes of the first process gas cooling zone (15); introducing a cooling medium into the shell side of the combo-boiler for cooling the shift reaction zone; removing a product gas (13) containing CO2 and H2 from the tubes of the shift reaction zone; removing a cooled first process gas and the cooling medium.    USE - For performing steam reforming and shift reaction for generation of hydrogen gas (claimed) used as a raw material in ammonia synthesis, methanol synthesis, and hydrogen chloride synthesis, hydrogen peroxide synthesis and is used in the production of oleochemical; for removing sulfur from hydrocarbon fuels such as gasoline and diesel.    ADVANTAGE - The method has increased the efficiency of hydrogen generation via steam reforming and low-temperature shift reaction, in particular with regard to temperature control and heat recovery; is performs isothermally and recovers a significant amount of heat, thus enhancing the overall efficiency of the system and reduces damage to the shift catalyst due to exposure to excessively high temperatures, and provides prolongs the life to the catalyst and provides better conversion.    DETAILED DESCRIPTION - Performing a shift reaction involves introducing a feed gas (1) comprising water (H2O) and carbon monoxide (CO) into a combo-boiler (14), where the combo-boiler comprising a shell and tube reactor having at least two tube zones and a common shell zone, where at least two tube zones include a shift reaction zone and a first process gas cooling zone, the tubes of the shift reaction zone containing a shift reaction catalyst and where the feed gas is introduced into the tubes of the shift reaction zone to convert H2O and CO into carbon dioxide (CO2) and hydrogen (H2); introducing a first process gas into the tubes of the first process gas cooling zone (15); introducing a cooling medium into the shell side of the combo-boiler for cooling the shift reaction zone and the first process gas cooling zone, where the cooling medium undergoes indirect heat exchange with the feed gas and the first process gas, where the shift reaction zone is operated under isothermal conditions; removing a product gas (13) containing CO2 and H2 from the tubes of the shift reaction zone; removing a cooled first process gas from the tubes of the first process gas cooling zone, and removing the cooling medium from the shell side of the combo-boiler. The cooling medium is not water. INDEPENDENT CLAIMS are included for following:    (1) performing steam reforming and a shift reaction involving subjecting a first hydrocarbon feed gas to desulfurization; introducing the resultant desulfurized first hydrocarbon feed gas and steam into a reforming chamber of a steam reformer, where the steam reformer comprising the reforming chamber and a separate burner or combustion chamber, where the desulfurized first hydrocarbon feed gas is subjected to steam reforming to produce steam-reformed gas comprising H2O and CO; combusting a second hydrocarbon feed gas and an oxygen-containing gas in the burner or combustion chamber of the steam reformer to provide indirect heat for steam reforming the desulfurized first hydrocarbon feed gas; introducing the steam-reformed gas comprising H2O and CO into a combo-boiler; introducing a first process gas into the tubes of the first process gas cooling zone; introducing steam-reformed gas comprising H2O and CO into the tubes of the shift reaction zone, where the steam-reformed gas undergoes a water-gas shift reaction in the shift reaction zone to convert H2O and CO into CO2 and H2; introducing a cooling medium into the shell side of the combo-boiler for cooling the shift reaction zone and the first process gas cooling zone, where the cooling medium undergoes indirect heat exchange with the first process gas and indirect heat exchange with the steam-reformed gas as it undergoes the water-gas shift reaction, where the shift reaction zone is operated under isothermal conditions; removing a product gas containing CO2 and H2 from the tubes of the shift reaction zone; removing a cooled first process gas from the tubes of the first process gas cooling zone; and removing cooling medium from the shell side of the combo-boiler; and    (2) an apparatus for hydrogen generation involving steam reforming and a water gas shift reaction, comprising a desulfurizer (3) containing a bed of desulfurization catalyst, where the desulfurizer having an inlet for introducing a hydrocarbon feed stream and an outlet for discharging a desulfurized hydrocarbon feed stream; a steam reformer comprising a burner chamber and a reaction zone, where the reaction zone containing a steam reformer catalyst, the reformer having a desulfurized hydrocarbon feed stream inlet for introducing a desulfurized hydrocarbon feed stream into the reaction zone, the feed stream inlet is in fluid communication with the outlet of the desulfurizer, the reformer further comprising a reformed product gas outlet for discharging reformed product gas containing H2, H2O, and CO, unit for introducing hydrocarbon fuel and an oxygen-containing gas into the burner chamber, and unit for removing flue gas from the burner chamber, either unit for combing desulfurized hydrocarbon feed stream with steam upstream of the desulfurized hydrocarbon feed stream inlet or a steam inlet for introducing steam into the reaction zone of the steam reformer; and a combo-boiler in the form of a shell/tube reactor, where the combo-boiler comprising a common shell zone, a first tube zone positioned within the common shell zone, and a second tube zone positioned within the common shell zone, the first tube zone having a first inlet for introducing reformed product gas and a first outlet for discharging heated reformed product gas, the first inlet being in fluid communication with the reformed product gas outlet of the steam reformer, the second tube zone having a second inlet for introducing heated reformed product gas and a second outlet for discharging shift reaction product gas containing H2 and CO2, where the tubes of the second zone containing a shift reaction catalyst, the common shell zone having a cooling medium inlet and a cooling medium outlet.    DESCRIPTION OF DRAWING(S) - The figure shows view of flow chart of process of shift reaction.    Feed gas (1)    Desulfurizer (3)    Product gas (13)    Combo-boiler (14)    Gas cooling zone (15)