NOVELTY - A carbon monoxide is prepared by performing isothermal methanation process, performing heat recovery process, and performing cooling process in which synthesized gas contains carbon monoxide, carbon dioxide, and hydrogen; carbon monoxide content is not greater than 0.5% (V), carbon dioxide content is not greater than 0.5% (V), or sum of carbon monoxide and carbon dioxide content is not greater than 0.5% (V); raw material gas after is detoxified first; total sulfur content is not less than 0.2 ppm; pressure is 0.5-6 MPa; heat exchanger and heater is heated to 150-350 degrees C; raw material gas enters into tube sleeve of isothermal methanation reactor (3).    USE - Preparation of methane (claimed).    ADVANTAGE - Preparation process increases depth the reaction, achieves multi-stage hat-insulation methanation reaction, does not need removing carbon dioxide, reduces operating costs.    DETAILED DESCRIPTION - A carbon monoxide is prepared by performing isothermal methanation process, performing heat recovery process, and performing cooling process in which synthesized gas contains carbon monoxide, carbon dioxide, and hydrogen; carbon monoxide content is not greater than 0.5% (V), carbon dioxide content is not greater than 0.5% (V), or sum of carbon monoxide and carbon dioxide content is not greater than 0.5% (V); raw material gas after is detoxified first; total sulfur content is not less than 0.2 ppm; pressure is 0.5-6 MPa; heat exchanger and heater is heated to 150-350 degrees C; raw material gas enters into tube sleeve of isothermal methanation reactor (3); tube sleeve has inlet tube and outer tubes; tube sleeve is filled with methanation catalyst; raw material gas reaches bottom of inner tube folded into outer tubes; methanation reaction produces methane gas and flows out through outer tube; isothermal methanation reactor has coolant circulation tube that controls temperature of methanation reaction which is 250-500 degrees C; from outlet of isothermal methanation reactor, gas enters heat exchanger (1) for heat recovery process; cooling process is done in cooler (5); and gas product enters dispenser (7) An INDEPENDENT CLAIM is included for isothermal methanation reactor which comprises cylinder, feeding port, raw material cabin, air chamber, and cooling medium region, and raw material gas chamber in which cylinder is set along radial direction of upper tube plate and lower tube plate with feeding port for raw material cabin; cylinder body of finished product air cabin is provided with discharging port; upper part of cylinder body cooling medium zone has coolant inlet and outlet; inner cylinder has inner pipe and outer pipe sleeve which is composed of tube; outer tube is hung on lower tube plate; upper end is opened and is connected with finished product storehouse; blind end extension extends to outside bottom; inner tube pipe is filled with methanation catalyst; annular inner wall of outer tube is cool medium area; inner pipe is hung on upper tube plate; upper end is opened and is connected with raw material cabin; and lower end is opened.    DESCRIPTION OF DRAWING(S) - The drawing shows a craft flow sketch of isothermal methanation reactor. (Drawing includes non-English language text).    Heat exchanger (1)    Isothermal methanation reactor (3)    Cooler (5)    Separator (6)    Gas bag (7)