Dealing with vapor liquid equilibrium (VLE) in hydrotreating reactors is a major obstacle to realistic process simulation. The present study focused on the modeling and simulation of a commercial light-cycle-oil (LCO) hydrotreater with rigorous vapor liquid equilibrium (VLE) calculations. The hydrotreater was represented as a one-dimensional plug-flow adiabatic reactor with interbed gas quenching. The model accounted for the hydrodesulfurization (HDS) and hydrodearomatization (HDA) reactions. VLE calculations were performed in situ with a calibrated flash calculation program developed in-house specifically for hydrogen petroleum systems. Significant differences in reactor temperature profiles, hydrotreating conversions, fluid rates, and phase compositions were observed between the simulations with and without VLE. Analysis of the effect of temperature indicated that reactor performance depends heavily on the axial temperature distribution, which is established based on the catalyst bed layout and the selection of bed inlet temperatures. It was observed that, although conversion levels were improved at higher inlet temperatures, there was an increase in quenching gas requirement, vaporization rate, and the formation of high-temperature zones toward the inlet of the reactor. In a similar manner, pressure was found to increase hydrotreating performance markedly but with an extensive heat release. Verification of plug-flow and full-catalyst-wetting conditions during the simulations confirmed that all criteria were satisfied in this study.