NOVELTY - An isothermal methanation process of coal-based natural gas involves methanation process which introduces purified synthesis gas into raw material gas pre-heater (1) and heat-insulation methanation reactor outlet heat gas exchange, pre-heating fine desulfurization groove to 280-300 degrees C, pre-heating gas, controlling total sulfur in synthesis gas to less than or equal to 30 ppb to protect methanation catalyst, entering synthesis gas into isothermal methanation reactor, entering waste heat boiler of 4 MPa medium-pressure steam, and reducing temperature to 280-300 degrees C after entering heat-insulation methanation reactor.    USE - Isothermal methanation process of coal-based natural gas for purifying coke oven gas synthetic natural gas (claimed).    ADVANTAGE - The method is energy-saving and environment-friendly.    DETAILED DESCRIPTION - An isothermal methanation process of coal-based natural gas involves methanation process which introduces purified synthesis gas into raw material gas pre-heater (1) and heat-insulation methanation reactor outlet heat gas exchange, pre-heating fine desulfurization groove to 280-300 degrees C, pre-heating gas, controlling total sulfur in synthesis gas to less than or equal to 30 ppb to protect methanation catalyst, entering synthesis gas into isothermal methanation reactor (3), entering waste heat boiler (5) of 4 MPa medium-pressure steam, reducing temperature to 280-300 degrees C after entering heat-insulation methanation reactor (6) in which unreacted carbon monoxide conversion to methane in outlet gas carbon monoxide content is 5-10 ppm, cooling, and entering gas-liquid separator (9) which separates 94-98% condensed water and high heat value is 37.38-38.37 MJ/Nm3.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the craft of coal-based natural gas. (Drawing includes non-English language text).    Raw material gas pre-heater (1)    Isothermal methanation reactor (3)    Waste heat boiler (5)    Heat-insulation methanation reactor (6)    Gas-liquid separator (9)