A method for monitoring polymerization processes can include reacting by polymerization a feedstock in the presence of a catalyst in a reactor to produce an effluent comprising a polymer and a solvent; measuring a density of the effluent; and calculating a monomer conversion rate and/or a polymerization rate for the polymerization based on the density of the effluent. A system for monitoring polymerization processes can include a reactor containing an effluent comprising a solvent, a polymer, and a monomer; a flash vessel fluidly coupled to the reactor to receive the effluent from the reactor; and an inline density meter fluidly coupled to the reactor, placed between the reactor and the flash vessel, and configured to measure a density of the effluent.