Molecular dynamics simulations of supercooled polymer melts are reported for long chains. This is the first simulation study at melt densities where the formation of chain-folded structures resembling the lamellae of polymer crystals is observed. Crystallization and subsequent melting temperatures are related linearly to the inverse lamellar thickness. Analysis of the single chain conformations in the crystal shows that most chains reenter the same lamella by tight backfolds. Simulations are performed with a mesoscopic bead-spring model including a specific angle bending potential. They demonstrate that chain stiffness alone, without an attractive inter-particle potential, is a sufficient driving force for the formation of chain-folded lamellae.