The present study addresses this challenge through NPT, NVT, and NVE molecular dynamics simulations of crystal nucleation in entangled polyethylene (PE) melts, specifically n-C 720 H 1442 melts. During an isochoric, isothermal NVT simulation, the heat released during crystallization is removed nearly instantaneously from the crystallizing system (e.g., on picosecond timescales). In an isolated (NVE) simulation, heat is unable to escape the system as it crystallizes. NVT and NVE simulations thus represent the two extremes that bound experimentally relevant heat-dissipation conditions. However, both NVT and NVE simulations are isochoric, not allowing for variations in system density as crystal formation proceeds. Therefore, isobaric, isothermal NPT simulations were also used to probe PE crystallization. In turn, it is herein demonstrated that molecular-level structural details of polymer crystal nucleation are not substantially affected by the heat released during early-stage crystal formation, nor variations is system densities as crystal nucleation proceeds. The observed phenomenology is connected to local structures and chain-level structural relaxations, which highlight that crystal nucleation under strongdriving force, long-chain conditions is qualitatively different from the visions of polymer nucleation afforded by previous studies. FIG. . Characteristics of crystal formation in PE melts under isolated conditions as realized through a NVE simulation. A) Temperature evolution during PE crystallization. B) Progression of the system towards crystalline PE as captured by the percentage of its constituent particles that are crystalline according to the P 2 order parameter as detailed in Sec. II.