This work aims to develop a rigorous model for industrial successive supercritical slurry-phase and gas-phase catalytic polymerization reactors of the Borstar bimodal polyethylene process. The model consists of thermodynamic modeling, multisite ZieglerNatta polymerization kinetics, and reactor modeling. The perturbed-chain statistical associating fluid theory equation of state (PC-SAFT EOS) with updated parameters allows an accurate description of the phase equilibria of the supercritical slurry-phase and gas-phase system in a wide range of temperatures and pressures. The model predictions of the process variables (i.e., residence time, molar ratio of H-2/C2H4 and H-2/1-C4H8 of each reactor) and polymer properties (i.e., molecular weight, comonomer content, molecular weight distribution (MWD)) agree well with the industrial plant data under multi-steady-state operation conditions. The model is also capable of simulating the effects on the MWD of bimodal polyethylene resulting from the hydrogen inflow rate in a supercritical slurry-phase loop reactor (SLR) and a gas-phase fluidized bed reactor (FBR), as well as the supercritical solvent propane inflow rate of the SLR.