Molecular dynamics simulations of the crystallization of eicosane have been carried out as an example of a low molecular weight n-alkane system and for a high molecular weight alkane as an analogue of a semicrystalline polyethylene. An alternative method for determining crystal domains and growing nuclei has been developed based on the eicosane system behavior. In the former case, density and crystal lattice parameters conformed well to previously reported experimental values. Moreover, crystallization and nuclei formation were monitored using a procedure developed herein. Analysis showed that metastable nuclei could be obtained at 250 K lasting appreciably long lifetimes of tens to hundreds of nanoseconds, which then exhibited an abrupt irreversible increase in size. The present study emphasizes the long quasi-equilibrium of these crystallizing systems even at moderate supercooling.