The reaction zone of a free-flow refractory lined partial oxidation gas generator is preheated by the complete combustion of fuel gas, supplied by means of a preheat burner, with air at atmospheric pressure. The preheat burner is replaced by the main process burner and the heating is continued by the complete combustion of said fuel gas and air at atmospheric pressure. The main burner is then made pressure tight to the reaction zone, and the heating is continued. When the temperature of the reaction zone reaches the temperature for ignition of the autothermal partial oxidation reaction of the liquid hydrocarbonaceous fuel and free-oxygen containing gas in the presence of a temperature moderator, fuel gas and air are cut-off, and are respectively replaced by a stream of said liquid hydrocarbonaceous fuel, optionally in admixture with a temperature moderator, and a separate stream of free-oxygen containing gas, optionally in admixture with a temperature moderator. After ignition, the partial oxidation reaction takes place in the reaction zone, and the gas generator is brought up to operating pressure. In the event of temporary shut down of the gas generator, the streams of liquid hydrocarbonaceous fuel, steam, and free-oxygen containing gas are shut-off and a stand by temperature may be maintained in the reaction zone to allow quick start-up by switching the feedstreams to the main burner back to fuel gas and air.