Among the diverse types of heat exchanger technology, the geometrical features of plate-fin exchangers provide the required flexibility for the incorporation of large number of streams in a single unit. The main design and operating issues related to multi-stream exchangers are linked to the complex thermal flows that take place inside the unit. Several simplifications have been proposed to reduce this complexity and derive design methodologies. One of these simplifications is the uniform channel heat load which gives rise to the uniform wall temperature assumption. This simplification assumes that at any plane cut normal to the flow of the fluids inside the exchanger, the temperature of the separating walls is the same. One way of practically achieving such condition is by appropriate surface selection. The thermal performance of secondary surfaces has been extensively studied and vast information on their thermo-hydraulic performance has been published. A design criterion that has emerged in the last decade is to engineer secondary surfaces to achieve the required thermal performance. Such approach is taken advantage of in this work so that the concept of uniform wall temperature in design can be effectively met. This paper shows the derivation of a surface design methodology. The approach is demonstrated on a case study using triangular shape fins.