NOVELTY - Methane-rich gas (36) is prepared by introducing synthesis gas (30) which is new or recycled gas to greater than or equal to 1 methanation reactor, extracting product gas (32) from 1st methanation reactor, cooling, extracting condensate and intermediate feed gas, heating feed gas, introducing feed gas to final methanation reactor in presence of catalytically active material to obtain methane-rich gas, cooling catalytically active material and methanation reactor by heat transfer, and extracting heat from final methanation reactor.    USE - Preparation of methane-rich gas (claimed).    ADVANTAGE - The preparation of methane-rich gas is isothermal, at low temperature, and has chemical equilibrium towards methane and high conversion rate.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) for methanation reactor (4) comprising 2 beds containing catalytically active material and heat exchange mechanism in between beds which involves introducing intermediate or final feed gas to 1st bed consisting of catalytically active material to obtain intermediate methane gas, cooling methane gas in heat exchange mechanism, introducing cooled gas to 2nd bed consisting of catalytically active material to obtain final methane-rich gas; and    (2) preparation of methane-rich gas processing device which involves arranging main body for receiving synthesis gas and guiding gas to methanation reactor containing 1st catalytically active material bed (14) to form product gas, arranging condenser (8) for receiving 1st product gas, cooling 1st product gas, and extracting liquid condensate (40) and intermediate feed gas, arranging final methanation reactor for receiving intermediate and final feed gas and guiding gas to methanation reactor containing 2nd catalytically active material bed (18), and arranging cooling reactor for cooling catalytically active material bed and gas by heat exchange.    DESCRIPTION OF DRAWING(S) - The drawing shows a process flow diagram of preparation of methane-rich gas.    Methanation reactor (4)    Condenser (8)    Heat exchanger (10)    1st catalytically active material bed (14)    Heat exchanger (16)    2nd catalytically active material bed (18)    Water softening mechanism (22)    Synthesis gas (30)    Product gas (32)    Methane-rich gas (36)    Liquid condensate (40)