Abstract: In methanizing CO and H.sub.2 containing charge gases, the charge gas is passed upwardly through a vertically extending fluidized bed reactor flowing, in turn, through a fluidized bed zone, a dust bed zone and a separation zone. From the separation zone, the gas is conveyed out of the reactor to a point of use. Coolant tubes are positioned in the reactor passing vertically through the fluidized bed zone, the dust bed zone and the separation zone for removing the heat generated in the reaction. An evaporable coolant, preferably water, is circulated through the coolant tubes. In a separate steam drum, the vapor pressure of the coolant is set for maintaining constant the temperature of the vaporizing coolant.