NOVELTY - Methanation of coal synthesis gas comprises water washing coal synthesis gas to remove dust and oil impurity, feeding and discharging the processed material through heat exchanger II and methanation reactor II outlet gas, then feeding and discharging through heat exchanger I and methanation reactor I outlet gas, mixing resulted product with water vapor and entering into the methanation reactor I, and then performing first level sulfur-tolerant methanation reaction in the presence of molybdenum based bifunctional catalyst.    USE - The method is useful for methanation of coal synthesis gas (claimed).    ADVANTAGE - The method ensures the methanation of coal synthesis gas with less energy consumption and good product quality in a simple and an economical manner.    DETAILED DESCRIPTION - Methanation of coal synthesis gas comprises water washing coal synthesis gas to remove dust and oil impurity, feeding and discharging the processed material through heat exchanger II and methanation reactor II outlet gas, then feeding and discharging through heat exchanger I and methanation reactor I outlet gas, mixing resulted product with water vapor and entering into the methanation reactor I, then performing first level sulfur-tolerant methanation reaction in the presence of molybdenum based bifunctional catalyst, passing the outlet gas through high pressure waste heat boiler I for heat recovery, feeding and discharging the synthesis gas through heat exchanger I and entering into methanation reactor II, then performing second level sulfur-tolerant methanation reaction in the presence of molybdenum based bifunctional catalyst, passing the outlet gas through high pressure waste heat boiler II for heat recovery, feeding and discharging the synthesis gas through heat exchanger II, further cooling the synthesis gas through an air cooler I and entering into a liquid separating tank I, separating the condensed liquid and gas phase at bottom and top of the liquid separating tank I respectively, cooling the gas through a depth cooler and entering into low-temperature methanol washing system to remove carbon dioxide and hydrogen sulfide, feeding and discharging the low-temperature methanol washing gas through heat exchanger III and entering into methanation reactor III, then performing third level sulfur-tolerant methanation reaction in the presence of nickel based catalyst to convert the synthesis gas into carbon monoxide and methane gas, passing the outlet gas through waste heat boiler III for heat recovery, feeding and discharging the resulted gas through heat exchanger III and low-temperature methanol washing system, cooling the resulted gas through an air cooler I and water cooler, entering the cooled gas into a liquid separating tank II, separating the condensed liquid and product gas at bottom and top of the liquid separating tank II, drying the product gas and compressing into natural gas pipeline network.