We report a numerical study of the free energy barrier for crystallization and melting of a single homopolymer chain. The simulations show that the free energy barrier separating the crystalline and molten states at the same free energy level strongly depends on the chain length. However, at a fixed temperature the barrier for single-chain crystallization is independent of chain length. This observation is in agreement with recent experiments on multichain bulk-polymer systems and can be understood theoretically if we assume that the primary nucleation of polymer crystals is determined by intramolecular nucleation. If we further assume that the subsequent growth of polymer crystals is controlled by two-dimensional intramolecular nucleation on the growth front, we can even account for the experimentally observed molecular segregation during crystal growth as well as the chain-length independence of the free energy barrier for secondary nucleation.