Abstract: A single stage methanization reactor and process are made to yield a product gas of high methane content as well as useful superheated steam without overheating the catalyst bed in which a synthesis gas containing, CO, CO.sub.2 and H.sub.2 is converted into product gas. A variety of dispositions of vaporizer and superheater portions of the cooling system in the catalyst bed, for which the temperature profiles of gas and coolant along the reactor length are shown and compared, illustrate the principles governing the cooling system for such a reactor. A small portion of the superheated steam is mixed with preheated synthesis gas for elimination of all or part of the carbon monoxide content before the synthesis gas is introduced into the methanization reactor.