Apparatus and method for cooling heated fluids, such as exhaust gases, flowing through a heat exchanger comprising one or more exhaust plenums and one or more coolant plenums, and providing increased coolant velocity in that portion of the coolant plenums contacting the inlet portion of the exhaust plenums. Local increased coolant velocity is provided by any means, including decreasing the area-in-flow of the coolant plenums wherein increased velocity is desired, shaping or baffling either or both inlet or outlet coolant tanks in fluidic contact with coolant plenums wherein increased velocity is desired, or a combination thereof.