In a synthesis gas methanation process, at least one first fraction of synthesis gas to treat is fed, together with steam, to a shift reactor where a shift reaction occurs; the gas flow produced in the shift reactor is then fed to a first methanation reactor where a methanation reaction occurs and then to further second methanation reactors in series, where further methanation reactions, performed with the addition of fresh synthesis gas which has not been subjected to the shift reaction.