A process control method for controlling a polyolefin polymerization process, such as a solution polyethylene process, which includes five steps. The first step is to flow a stream of a solvent into a polyolefin reactor, such as flowing a stream of heptane solvent into the reactor at essentially a constant rate. The second step is to add a principal monomer to the stream of solvent, such as by adding ethylene at a controllable rate to the stream of solvent. The third step is to add a co-monomer to the stream of solvent, such as by adding 1-octene at a controllable rate to the stream of solvent. The fourth step is to determine the concentration of the principal monomer and the concentration of the co-monomer in the stream of solvent by high resolution multi-wavelength vibrational spectroscopy analysis, such as by Fourier transform infrared spectroscopy system having a spectral resolution of 4 wavenumbers which analyzes at 1909 wavenumbers for ethylene, at 1829 wavenumbers for 1-octene and at 2120 wavenumbers for a reference signal. The fifth step is to control the rate of addition of the principal monomer, the rate of addition of the comonomer or preferably the rate of addition of both the principal monomer and the co-monomer according to the determination of the fourth step to obtain the desired concentration of ethylene and 1-octene in the solvent so that, for example, the density of the polyolefin product can be controlled.