A rigorous method that takes into consideration stacking pattern is developed and presented for the thermal performance prediction of multistream plate-fin heat exchangers. The method also accounts for transverse conduction of heat through fins. Some published sizing and rating methods dealing with stacking patterns are reviewed. It is shown that rating methods based on a constant surface temperature give optimistic predictions. Transverse heat conduction was found beneficial to the performance of the exchanger. It was found that, in general, patterns that reduced variation of surface temperature through the depth of the stack improved performance. Application of the proposed method is discussed with examples. A procedure for the process design of multistream plate-fin heat exchangers is outlined.