We developed a theoretical model of linear ultrahigh molecular weight polyethylene (UHMWPE) homogeneous polymerization. We considered polymerization to be living and occurring in a poor solvent. We derived the dependency of the entanglement length on the chain length during this process. We assessed how the rate of polymerization and the concentration of initiators affect entanglement of chains. Theoretical predictions were supported by the molecular dynamics computer simulations of the coarse-grained model of polyethylene. The computer model implemented living polymerization of linear chains, poor solvent conditions, formation of entanglements, and crystallization of growing chains. Polyethylene chains were modeled by using specific angular potential with the three energy minima. Our theory and the simulation results pointed out that there are two stages of homogeneous polymerization. At first, the chains grow independently. When the chains become long enough, they start to entangle, and the entanglement length starts to decrease. We also observed in simulations the existence of the third stage, when the entanglement length grows with the chain length at the very high conversion degrees. Theoretical predictions and the simulation results showed that a decrease of the concentration of initiators leads to a decrease in the entanglement density in the resulting semicrystalline UHMWPE sample. Our theory also predicted the existence of an optimal reaction rate for obtaining semicrystalline UHMWPE samples with the highest possible entanglement length at a given concentration of initiators. Our work could be a guide how to obtain low-entangled UHMWPE samples by tailoring the reaction conditions.