A kinetic model is developed for predicting the polymerization rate, chain-branching characteristics, and molecular weight from ethylene polymerization over homogeneous and supported nickel diimine catalysts. Seven dimensionless parameter groups are able to describe the polymer properties and kinetic behavior over a broad range of temperatures and pressures. Relative to existing models in the literature, the proposed model is simpler, requires fitting of fewer kinetic parameters, and can be more easily extrapolated over a wide range of reaction conditions. As such, the model can be more readily incorporated into reactor process models for performing dynamic simulations. Experimental data from the literature are used to fit the model kinetic parameters. The model is able to describe the kinetic behavior of both homogeneous and heterogeneous nickel diimine catalysts, and the results compare favorably with existing models in the literature.