NOVELTY - The method comprises producing the methane gas by catalytic conversion of the feed gas in a methanation unit (100), transferring the heat formed during the catalytic conversion from the methanation unit to a vapor generating unit (300) by a heat transfer unit (200), and producing vapor in the vapor generating unit, from a liquid present in the unit by the transferred heat, where the methanation step and the heat transferring steps are carried out by a number of parallelly switched functional units (A, B, C) that are formed from the methanation unit and the heat transfer unit.    USE - The method is useful for combined production of vapor and methane gas from a feed gas.    ADVANTAGE - The method produces vapor and methane gas in a simple, synergistic and rapid manner.    DETAILED DESCRIPTION - The method comprises producing the methane gas by catalytic conversion of the feed gas in a methanation unit (100), transferring the heat formed during the catalytic conversion from the methanation unit to a vapor generating unit (300) by a heat transfer unit (200), and producing vapor in the vapor generating unit, from a liquid present in the unit by the transferred heat, where the methanation step and the heat transferring steps are carried out by a number of parallelly switched functional units (A, B, C) that are formed from the methanation unit and the heat transfer unit and are modularly interchangeable. The methanation unit comprises a methanation reactor and the catalysts are activated by heat, where the heat is carried by the feed gas itself or supplied to the feed gas through the methanation reactor. An INDEPENDENT CLAIM is included for a modular vapor-methane gas reactor (10) for combined production of vapor and methane gas from a feed gas.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of a modular vapor-methane gas reactor.    Modular vapor-methane gas reactor (10)    Methanation unit (100)    Heat transfer unit (200)    Vapor generating unit (300)    Functional units. (A, B, C)