NOVELTY - A heat exchanger apparatus comprises a conduit for extracting a volume of a cooling medium from a cooling medium compartment, and a cooling mechanism provided in an upstream end of a heat exchanger tube and comprising supply mechanism to supply a mixture of the extracted cooling medium and part or all of the fresh cooling medium as supplied to the vessel along the exterior of the upstream end of tube.    USE - For cooling hot gas obtained from gasification of (hydro)carbon-containing fuel used in petroleum industry, by passing the hot gas through a tube having main tubular part and upstream tubular part, cooling the exterior of the main tubular part by an evaporating liquid cooling medium flowing inside a vessel and around the tube, and cooling the upstream tubular part by passing fresh liquid medium (claimed).    ADVANTAGE - The invention has better defined cooling and improved equipment lifetime and improved reliability without having the disadvantages of the prior art designs.    DETAILED DESCRIPTION - A heat exchanger apparatus comprises a vessel (2) provided with a cooling medium compartment (7), an inlet to supply fresh cooling medium, and a outlet for discharge of used cooling medium; and a conduit for extracting a volume of the cooling medium from the cooling medium compartment. The upstream end of the tube is provided with a cooling mechanism comprising supply mechanism to supply a mixture of the extracted cooling medium and part or all of the fresh cooling medium as supplied to the vessel along the exterior of the upstream end of tube. The vessel is provided with an inlet (9) for hot gas and an outlet (5) for cooled gas, at least one heat exchange tube (4) connecting the inlet for hot gas, and the outlet for cooled gas positioned in the cooling medium compartment. The tube is mounted at least at or near its upstream end in a tube plate. An INDEPENDENT CLAIM is also included for configuration of a partial oxidation reactor and the heat exchanger apparatus connected at their lower end by a horizontal duct. In the duct, the upstream end of the heat exchanger and its cooling mechanism are positioned.    DESCRIPTION OF DRAWING(S) - The figure shows a sectional view of a heat exchanger.    Vessel (2)    Heat exchanger tub (4)    Gas outlet (5)    Cooling medium compartment (7)    Gas inlet (9)