Method for methanizing charge gases containing CO and H.sub.2 in a fluidized bed reactor containing an upwardly extending fluidized bed zone containing a powdered catalyst and a separation zone located above the fluidized bed zone, flowing the charge gases upwardly through the fluidized bed zone from the lower end thereof and reacting the charge gases with the powdered catalyst in the fluidized bed, removing the heat generated in the reaction by an evaporating coolant and passing the evaporating coolant through the fluidized bed zone in indirect heat transfer relation with the powdered catalyst therein, adjusting the vapor pressure of the coolant before introducing it into the fluidized bed zone for maintaining a desired boiling temperature of the coolant in the fluidized bed zone, wherein the improvement comprises, maintaining an upwardly extending dust bed zone within the reactor between the upper end of the fluidized bed zone and the separation zone in the fluidized bed reactor with the powdered catalyst being present in the upwardly flowing charge gases in the dust bed zone as a cloud of powdered catalyst, reacting the charge gases with the cloud of powdered catalyst in the dust bed zone, separating the powdered catalyst from the charge gas within the separation zone whereby the separation step is completely effected within the reactor, removing the heat generated in the reaction from the dust bed zone and from the separation zone by flowing the evaporating coolant from the fluidized bed zone in indirect heat transfer relation serially through the cloud of powdered catalyst in the dust bed zone and then the separation zone, maintaining the temperature of the evaporating coolant in the range of 60.degree. to 80.degree. C. below the temperature in the fluidized bed zone and in a corresponding range of 50.degree. to 70.degree. C. below the temperature in the dust bed zone with the pressure of the evaporating coolant being adjusted for establishing the boiling temperature of the coolant.