An apparatus for cooling hot gas comprises: (i) a vertical elongated vessel provided with a cooling medium compartment comprising in use a first cooling medium, an inlet to supply fresh first cooling medium and a outlet for discharge of used first cooling medium, the vessel further provided with an inlet for hot gas and an outlet for cooled gas, at least one heat exchange tube, which heat exchange tube is positioned in the cooling medium compartment (and fluidly connects the inlet for hot gas and the outlet for cooled gas, (ii) said cooling medium compartment further comprising a vertical and centrally positioned open ended downcomer wherein said heat exchange tube is positioned in a space between the downcomer and the vessel wall, (iii) and wherein the heat exchanger tube is mounted at its upstream end in a tube plate and wherein the tube plate is provided with means to supply a second cooling medium to the exterior of the upstream end of the heat exchanger tube, means to discharge the used second cooling medium, means to reduce the temperature of the used second cooling medium and means to recycle the second cooling medium to the exterior of the upstream end of the heat exchanger tube.