We have studied the structure and free energy landscape of a semiflexible lattice polymer in the presence of the surface of a polymer crystal. At low temperatures coexistence of two-dimensional integer-folded crystals is observed. As the temperature is increased there is a transition from these crystalline configurations to a disordered coil adsorbed onto the surface. The polymer then gradually develops a three-dimensional character at higher temperatures. We compute the free energy as a function of increasing crystallinity and compare with the free energy profiles assumed by the Lauritizen-Hoffman surface nucleation theory of polymer crystallization. Our free energy profiles exhibit a "sawtooth" structure associated with the successive formation of chain folds. However, in the early stages of crystallization our profiles significantly deviate from those assumed by surface nucleation theory because the initial nucleus is not a single stem but two incomplete stems connected by a fold. This finding has significant implications for the theoretical description of polymer crystallization.