To improve the transportation efficiency and reduce the supply cost, the liquefaction becomes an important tech-nology to store and transport the natural gas. During the liquefaction, the various components (e.g propane,ethane, methane etc) undergo fractional condensation phenomenon due to their different boiling points. This means that when one component condenses, others play a role of non-condensable gas (NCG). In order to reveal the influence mechanism of NCG on this condensation process, a numerical method was employed in this paper to study the condensation characteristics of three non-azeotropic binary hydrocarbon vapor mixtures, namely the propane/methane (80％-95％), ethane/methane (65％-85％) and methane/nitrogen (2％-13％) mixtures, on a vertical plate. The model was proposed based on the diffusion layer model, and the finite volume method was used to solve the governing equations. A user defined function was developed by cell iterative method to obtain the source terms in the condensation process. The numerical results show that the gas phase boundary layer formed by the NCG becomes the main resistance to the reduction of heat transfer coefficient. And for the above three mixtures, there is a negative correlation between the NCG concentration and the heat transfer coef-ficient. Meanwhile, the results show a good agreement with the experimental data, meaning that the proposed model is reliable. Three mixtures within same non-condensable mole fraction of 20％ were also investigated,indicating that the mixtures with a higher binary hydrocarbon molecular ratio have a lower heat transfer coeffi-cient. As a result, the presence of the lighter NCG contributes to a thicker boundary layer.