TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Apparatus: The auxiliary lines are provided with respective flowrate adjusting groups operated by a control system as a function of temperature values detected at the outlet of each second methanation reactor. The apparatus comprises a control system operatively connected to the gas feed circuit and to the steam feed circuit and comprises temperature controllers associated to respective adjusting groups and configured so as to detect the temperature of the gas flow circulating in the gas feed circuit in predetermined positions, particularly along the primary line at the outlet of each methanation reactor, and accordingly control the associated adjusting groups. The first controller detects the temperature of the gas flow at the outlet of the first methanation reactor and controls the adjusting group of a steam inlet line for adjusting the flowrate of steam that is fed to the shift reactor. The apparatus comprises heat exchangers arranged along the primary line upstream of the shift reactor and/or a respective methanation reactor for cooling the gas flows entering each of the reactors. The heat exchangers are connected to the steam feed circuit and produce at least part of the steam fed to the shift reactor.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The flowrate of fresh synthesis gas fed to each second methanation reactor is adjusted as a function of the temperature of the gas flow detected at the outlet of the same methanation reactor. The gas flow coming out from the first methanation reactor is fed entirely to the second methanation reactors in series. The process involves a step of adjusting the amount of steam fed to the shift reactor as a function of the temperature of the gas flow detected at the outlet of the first methanation reactor. The shift reactor is fed with the amount of steam necessary for the shift reaction and with an additional amount of steam necessary to control the exothermicity of the methanation reaction in the first methanation reactor. The shift reaction is performed so as to reduce the CO content of the gas flow that feeds the first methanation reactor, and moreover to produce an amount of carbon dioxide (CO2) that acts, in the subsequent methanation reactions, as a reagent and as an inert for containing the exothermicity of the methanation reactions, together with a water (H2O) residual of the shift reaction. The shift reaction is performed in excess of H2O. The process involves cooling steps of the gas flow entering the shift reactor and/or at least one methanation reactors. The cooling steps produce at least part of the steam fed to the shift reactor. EXAMPLE - No suitable example given.