Described herein are methods for continuous solution polymerization. The method may comprise polymerizing one or more monomers and comonomers in the presence of a solvent in a polymerization reactor to produce a polymer solution; determining the composition of the polymer solution exiting the polymerization reactor in an on-line fashion; determining at least one of the critical pressure or critical temperature; comparing the critical pressure and/or critical temperature to the actual temperature of the polymer solution and the actual pressure of the polymer solution; heating or cooling the polymer solution to a temperature within 50 °C of the critical temperature; and passing the polymer solution through a pressure letdown valve into a liquid-liquid separator, where the pressure of the polymer solution is reduced or raised to a pressure within 50 psig of the critical pressure to induce a separation of the polymer solution into two liquid phases.