NOVELTY - Reactor (50) comprises: a reactor body (52) having an upper gas discharge part (54) for emitting methane-containing gas produced by reacting a reaction gas including carbon monoxide and hydrogen; a gas supply pipe (51) for supplying the reaction gas from upper part to the lower part, where the gas supply pipe is spaced apart from an reactor body inner wall and inserted into the interior of the reactor body; and multiple internal cooling tubes for flowing coolant in a spaced area between the gas supply pipe and the reactor body inner wall.    USE - The reactor is useful for synthesizing methane (claimed).    ADVANTAGE - The reactor minimizes number of reactors to producing greater than or equal to 80% of methane, efficiently extracts reaction heat in the catalyst layer, reduces the maximum temperature of the catalyst layer, and prolongs service life of the catalyst.    DETAILED DESCRIPTION - Reactor (50) comprises: a reactor body (52) having an upper gas discharge part (54) for emitting methane-containing gas produced by reacting a reaction gas including carbon monoxide and hydrogen; a gas supply pipe (51) for supplying the reaction gas from upper part to the lower part, where the gas supply pipe is spaced apart from the reactor body inner wall and inserted into the interior of the reactor body, the lower ends of the gas supply pipe is open and connected to the reactor body, and the reaction gas is transferred to the upper part from the lower part through the separated space between the gas supply pipe and the reactor body inner wall, and the reactor body is provided with a catalyst layer (53) packed with a catalyst to generate methane-containing gas by reacting with the reaction gas; and multiple internal cooling tubes for flowing coolant in a spaced area between the gas supply pipe and the reactor body inner wall.    DESCRIPTION OF DRAWING(S) - The figure shows a cross sectional view of the reactor for synthesizing methane.    Reactor (50)    Gas supply pipe (51)    Reactor body (52)    Catalyst layer (53)    Gas discharge part (54)