NOVELTY - A synthesis gas methanation process, involves: a shift reaction step, in which first fraction of synthesis gas to treat is fed, together with steam, to a shift reactor; a first methanation step, in which the gas flow produced in the shift reactor is fed to a first methanation reactor; at least one further methanation steps performed in sequence in respective second methanation reactors in series; and further methanation steps are performed by adding fresh synthesis gas, which has not been subjected to the shift reaction.    USE - As synthesis gas methanation process (claimed) used to produce methane.    ADVANTAGE - The process and an apparatus allows an accurate control of the methanation reaction in a simple and fully effective manner; allow to control the high exothermicity of the methanation reactions without recurring to heavy recirculation or other complicated solutions; and as a matter of fact, the control of the methanation reaction is carried out by introducing in the corresponding methanation reactors flows having a high content of inert, obtained in a preliminary shift step with the addition of steam, by feeding fractions of fresh synthesis gas (which has not passed through the shift reactor) in the possible following methanation reactors.    DETAILED DESCRIPTION - A synthesis gas methanation process, involves: a shift reaction step, in which at least a first fraction of synthesis gas to treat is fed, together with steam, to a shift reactor where a shift reaction occurs; a first methanation step, in which the gas flow produced in the shift reactor is fed to at least a first methanation reactor where a methanation reaction occurs; at least one further methanation steps performed in sequence in respective second methanation reactors in series; and where the further methanation steps are performed by adding fresh synthesis gas, which has not been subjected to the shift reaction. An INDEPENDENT CLAIM is included for a synthesis gas methanation apparatus, comprising: a shift reactor, connected to a primary line of a gas feed circuit and a steam feed circuit, so as to be fed with synthesis gas by means of the primary line and with steam by means of the steam feed circuit to perform a shift reaction; at least one first methanation reactor where a methanation reaction occurs and arranged downstream of the shift reactor along the primary line; at least one further second methanation reactors, arranged in series downstream of the first methanation reactor along the primary line; the apparatus is characterised in that the gas feed circuit comprises at least one auxiliary lines that feed respective second methanation reactors with flows of fresh synthesis gas that do not pass through the shift reactor.