Thin films of a mixture of two kinds of short polyethylene chains, n-tricontane and n-tetracontane, are simulated both at high and low temperatures by a dynamic Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Films are obtained from an equilibrated model for the melt by increasing one of the three periodic boundaries to a very large value. The melting temperatures (T-m) of the two pure components in the simulation are obtained from a sharp transition of various parameters, such as the probability of trans conformation of C-C bonds, orientation order parameters, energies, etc. At high temperature, 420 K (above T-m of the two pure components), the chains have a trend to separate with each other. The shorter chains are enriched on the free surface due to their larger fraction of chain ends. At low temperature (below T-m of the two pure components), two situations exist, which depend on the prior history of the mixture. If the quench takes place from a homogeneous mixture the chains crystallize together with almost the same density profile except for a large fluctuation in the bulk region of the thin film. On the other hand, if the quench takes place from the equilibrated thin film at 420 K, the shorter chains remain enriched on the surface, and the segregation of the shorter chain is enhanced.