Although the determination of heat exchanger performance based on the log-mean-temperature-difference approach or the temperature effectiveness approach had been well established, the understanding of the fluid temperature variation along the heat exchanger was still essential in some situations in which the fluid properties were expected to vary substantially across the heat exchanger. In this study, two normalized temperature surpluses were employed to predict the temperature profiles of the hot and cold fluid streams. With specified capacitance rates for the hot and cold fluid streams and the overall heat transfer value of the heat exchanger, unified profiles for the normalized temperature surplus along the heat exchanger could be formulated irrespective of the actual fluid entering temperature. The differences between the mean normalized temperature surplus and that based on the arithmetic mean between the fluid inlet and outlet increased when the capacitance rates of the two fluid streams departed more. By comparing the simulated performances of a sample waste heat recuperator based on both mean-temperature approaches to evaluate the respective average fluid properties with those using a detailed numerical approach over a range of entering fluid temperatures, it was found that the mean normalized temperature surplus was better.