NOVELTY - The production of synthesis gas from feedstock, comprises adding feedstock to water vapor for steam reforming in multitubular reactor-exchanger to form first synthesis gas, heating reaction tubes (38) by circulating a heating fluid in a shell (37) to form second synthesis gas, and producing the gas from steam reforming effluent and optionally from the heating fluid. The multitubular reactor exchanger has reaction tubes containing steam reforming catalyst and the shell. The heating fluid comprises first combustion gas flow of the second feedstock.    USE - Useful to produce synthesis gas from feedstock. The synthesis gas is useful to produce hydrogen and liquids by chemical conversion.    ADVANTAGE - The process is reliable, compact, coercive and cost effective.    DETAILED DESCRIPTION - The production of synthesis gas from feedstock, comprises adding feedstock to water vapor for steam reforming in multitubular reactor-exchanger to form first synthesis gas, heating reaction tubes (38) by circulating a heating fluid in a shell (37) to form second synthesis gas, and producing the gas from steam reforming effluent and optionally from the heating fluid. The multitubular reactor exchanger has reaction tubes containing steam reforming catalyst and the shell. The heating fluid comprises first combustion gas flow of the second feedstock. The feedstock comprise hydrocarbons and optionally recycled compounds. Circulating the heating fluid in the reactor-exchanger for heating the reacting tubes. Mixing the fluid with third feedstock and an oxygen in a complementary combustion zone to increase the temperature of the first flow of the obtained mixture, which circulates in the reactor exchanger for complementary heating of the tubes before the mixture leaves from the exchanger. The complementary combustion zone (32, 34) is arranged inside the reactor exchanger shell. Inner side of the reactor-exchanger shell has a third feedstock and the oxygen, which are injected into the shell in an upstream of a catalytic combustion zone under turbulence conditions or temperature of mixture is lower than the temperature or combustion occurs with the flame. The flow comprises the first feedstock added to the water vapor that circulates in the reactor exchanger shell, and the catalytic combustion zone is placed at the end of the first half of the reactor-exchanger shell. The heating fluid completely circulates through co-flow of the first feedstock. The complementary combustion zone (42, 44) is arranged outside of the reactor exchanger shell. The heat is exchanged into the flow at the exit of the reactor-exchanger shell for pre-steam reforming of the first feedstock before water vapor is entered into the reactor-exchanger. The temperature of the steam reforming effluent and the fluid in the exit of the reactor-exchanger are 800-950degreesC and 850-1150degreesC respectively. The flow comprises the first feedstock added to the water vapor, which is preheated between 500-700degreesC before the water vapor is entered into the reactor-exchanger shell. The heating fluid is a hydrocarbon combustion gas with air under a pressure of 0.4-4 MPa. The heating fluid is a flow of partial oxidation of hydrocarbon with air under a pressure of 0.5-12 MPa, without nitrogen and comprises water vapor. An INDEPENDENT CLAIM is included for a production of hydrogen from synthesis gas.    DESCRIPTION OF DRAWING(S) - The diagram shows a device for producing synthesis gas:    Complementary combustion zone; (32, 34)    Spacers; (36)    Shell; (37)    Reaction tubes. (38)