A novel multi-environment CFD micromixing model is used to describe the small-scale mixing of chemical species inside a tubular low-density polyethylene (LDPE) reactor under di!erent operating conditions. The model is coupled with a comprehensive kinetic scheme describing ethylene polymerization that includes kinetic mechanisms describing polymer properties and ethylene decomposition. The simulation results show that imperfect mixing between initiator and monomer reduces monomer conversion and increases the polydispersity index. Insu$cient micromixing also causes local hot spots, which may lead the reactor to thermal runaway. Thus, the small-scale mixing has a signi"cant impact on the reactor stability. The study not only illustrates the importance of mixing e!ects on LDPE polymerization but also provides important insights into the physical phenomena occurring inside the reactor, which are extremely helpful in evolving a criteria for stable operation of the reactor while controlling the product quality in the plant-scale tubular LDPE reactor. Compared to full probability density function (PDF) methods used in the literature for similar studies, the multi-environment CFD micromixing model o!ers a computationally highly e$cient description of the turbulent reacting #ow inside the LDPE reactor.