The addition of carbon-centered radicals to ethene, which are important in free-radical polymerization processes, are studied from a theoretical point of view. Experimental data for the rate constants are only available for the addition of methyl, ethyl, propyl and butyl radicals. The latter reactions are taken as model systems to derive a cost-effective method for the addition of alkyl radicals to ethene. The proposed model must be accurate and computationally feasible for additions in which larger radicals are involved. Accuracy is validated by direct comparison of theoretical and experimental rate constants in the temperature range from 300 to 600 K. A variety of electronic-structure methods were tested ranging from Hartree–Fock and post-Hartree–Fock methods to pure and hybrid density functional theory methods. Molecular partition functions were refined by treating large amplitude vibrations beyond the harmonic oscillator approximation.