Simulation of the evaporation process of a naphtha feedstock in the convection section of a steam cracking furnace is reported in this article. A two-step evaporation model was developed to determine component changes in the vapor and liquid phases. First, a feedstock-simplifying method, based on the work of Eckert and Vanek (Comput. Chem. Eng. 2005, 30, 343-356), was developed in which the original hydrocarbon mixture is represented by a system of real components. Second, the component distribution was determined by vapor liquid equilibrium theory, using an activity coefficient method. The group contribution method (GCM) was employed to calculate the interaction parameters in the activity coefficient method. Using a mechanistic heat-transfer method, a comparison was made between the evaporation model developed in this work and a simplified evaporation model. The simulation results are in good agreement with industrial data. Some parameter profiles for the heat-transfer process are provided, as are the flow patterns of the naphtha feedstock.