Abstract: A multitubular heat exchanger used as a feedwater heater in a power generating plant and the like includes a shell, a high temperature tube bundle and a low temperature tube bundle composed of a multiplicity of U-shaped heat transfer tubes arranged in the shell and a vent tube interposed between the two tube bundles. A laterally-closed flow passage is defined in at least one of the high temperature tube bundle and the low temperature tube bundle to induce part of a heating medium flowing between the shell and the tube bundle to flow to a position deep in the tube bundle and near the vent tube, so that a noncondensable gas stagnating zone formed in the tube bundle can be moved near to the vent tube and noncondensable gas can be removed from the shell by extraction through the vent tube.