NOVELTY - Methane is synthesized by removing benzene, naphthalene, heavy hydrocarbon and sulfide in coke oven gas by using purifying system, compressing the purified coke oven gas to 0.5-2 MPa, heating at 300-400 degrees C, adding water vapor based on weight of methane (CH4) gas in the coke oven gas, and mixing; performing first methanation reaction at 330-400 degrees C in the presence of nickel catalyst, cooling, performing second methanation reaction, cooling, and performing third methanation reaction; and introducing the obtained synthesized gas in a pressure swing adsorption device, and separating methane.    USE - Method of synthesizing methane (claimed).    ADVANTAGE - The method can generate natural gas composition having high methane content, low content of impurities, and high heat energy. It is environmental-friendly, saves energy, and provides alternative way of developing new form of energy. The proper addition of water vapor in the methanation reaction inhibits long methanation reaction to reduce the escape of heat energy during the reaction process and cooling of gas and prevent the catalyst from being inactivated, thus the method ensures a continuous and normal synthesizes of methane.    DETAILED DESCRIPTION - Methane is synthesized by removing benzene, naphthalene, heavy hydrocarbon and sulfide in coke oven gas by using purifying system, compressing the purified coke oven gas to 0.5-2 MPa, heating at 300-400 degrees C, adding water vapor based on weight of methane (CH4) gas in the coke oven gas where the mole ratio of water vapor and CH4 is 0.2-1:1, and mixing; performing first methanation reaction at 330-400 degrees C in the presence of nickel catalyst, cooling, performing second methanation reaction, cooling, and performing third methanation reaction; and introducing the obtained synthesized gas in a pressure swing adsorption device, and separating methane.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a method for synthesizing methane. (Drawing includes non-English language text.)