In a high-temperature high-pressure high-density polyethylene (HDPE) solution polymerization process it is imperative that the solution remains a super-critical single-phase throughout the reaction and adsorption steps to prevent severe operational problems. This work describes how an on-line phase equilibrium predictor was developed to alert plant operators of processing conditions that can lead to two-phasing, crossing the polymer solubility boundary from a single liquid phase to two liquid phases during steady-state operation of the plant. The dimensionless form of the Benedict-Web-Rubbin equation of state was used to develop the predictor. A comparison of the model predictions to plant observations for different sets of processing conditions shows that the average model prediction error for two-phasing pressures were in the range from 3.45% to -4.37% for cyclohexane and from -3.09% to 5.28% for hexane.