Crystallization and melting are fundamental thermal processes in crystalline polymers In contrast to recent considerable efforts to simulate molecular processes of crystallization investigations of melting in polymer crystals are still rare The present report deals with molecular dynamics modeling of melting in chain folded lamellae of short polyethylene like molecules made of 100 methylene units We observe two types of melting with distinct molecular mechanisms equilibrium melting at the fold surface and nonequilibrium melting at the growth front The former fold surface melting is very quick and reversible giving increasingly thinner lamellae at higher temperatures Large molecular mobility in the crystal is suggested to be the essential prerequisite for this specific mode of melting The latter type of melting at the growth front is much slower and irreversible, and it becomes apparent right after the end of rapid surface melting It is found that the rate of the irreversible melting shows intriguing similarity to the supercooling dependence of the crystallization rate Large dynamic heterogeneity in the chain folded crystals is also found, the molecular mechanism of which is closely correlated to the chain conformation at the fold surface The presence and specific time dependence of solid state diffusion between different phases (crystalline phase, amorphous phase at the fold surfaces and isotropic melt phase) are also discussed