Abstract: A plate-fin type of heat exchanger (400) facilitates exchange of heat between two process streams (S1, S2) e.g. high pressure methane and seawater. It comprises a matrix (M) of heat exchange plate elements (200') arranged side-by-side, flow passages (401) for the seawater process stream (S2) being defined between adjacent plate elements. The plate elements (200') are a high-integrity diffusion bonded sandwich construction comprising two outer sheets (101, 103--FIG. 3) and a superplastically expanded core sheet structure (102--FIG. 3) between the two outer sheets. The sandwich construction provides flow passages (117') for the methane process stream. Adjacent plate elements (200') are held in position relative to each other by serrated racks (403) which engage the edges of the plate elements.