A plastic heat exchanger has a shell with tanks disposed side-by-side. First and last distribution tanks straddle heat transfer tanks. The shell has a plurality of flow gaps, to allow coolant to flow between adjacent tanks. Spacers hold the flow gaps open.Upper and lower flanges are adhesively attached to the shell. Flange holes align with the tanks. Each flange hole has an annular wall encircling one of the tanks to preclude the shell from expanding.Upper and lower manifolds are adjacent the flanges, and have chambers disposed side-by-side for each heat transfer tank. Baffles are adapted to be removably received between the chambers to selectively block flow.A plurality of tubes extends between upper and lower tube headers, comprising a tube bundle, removably received within each of the heat transfer tanks.A header O-ring is between each header and flange. A seal plate is disposed against each flange. Each seal plate has seal plate holes in alignment with the flange holes, defining a hole pair. Each hole pair has a groove to receive the header O-ring. Each manifold and seal plate has a manifold seal.The shell, the manifolds, the headers, and the flanges, are constructed of a polymer. The tubes are constructed of metal materials having efficient heat transfer properties.