A heat exchanger for condensing into liquid phase a vapor containing non-condensable gas components through heat exchange with a heat exchanging medium has a vessel having a vapor inlet for receiving the vapor containing the non-condensable gas components and a discharge port for the non-condensable gas components. The heat exchanger further has a tube nest disposed in the vessel and having a plurality of tubes through which the heat exchanging medium flows. The vapor containing non-condensable gas components flows towards the discharge port across the tube nest so as to be condensed into liquid phase through heat exchange with the heat exchanging medium flowing through the tubes, so that the non-condensable gas components are separated from the liquid phase and flow towards the discharge port. The tube nest is constructed to create a change in the dynamic pressure of flow of the vapor towards the discharge port to cause such a static pressure distribution in the heat exchanger that a lower static pressure is established in a region where the discharge port is provided than in a region where the vapor inlet is provided.