A method for controlling a gas turbine power plant, a gas turbine power plant, a method for controlling a carbon-containing fuel gasifier, and a carbon-containing fuel gasifier that can keep constant the amount of heat generated from a synthetic gas produced by the carbon-containing fuel gasifier. The gas turbine power plant includes a coal gasifier including a coal gasifier unit that gasifies a fuel containing carbon to produce a synthetic gas and a water-cooled wall duct disposed on the coal gasifier unit and to which a coolant, i.e., water, is directed; a gas turbine combustor that combusts the synthetic gas to produce combustion gas; a gas turbine rotated by the combustion gas produced by the gas turbine combustor; and a generator that generates electrical power. The amount of fuel fed to the coal gasifier is controlled depending on the amount of heat absorbed by the coolant directed to the water-cooled wall duct.