Quantifying the effect of gas-liquid mass-transfer resistance is vital in studies aimed at extracting intrinsic kinetic parameters. This paper describes methods to evaluate this effect in a reactor used for slurry polymerization. Propene was polymerized using a modern Ziegler-Natta catalyst dispersed in decane. The stirring rate was changed during polymerization, and the observed monomer feed rates were analyzed using methods based on steady-state and dynamic mass balances to obtain mass-transfer coefficients. It was found that the system investigated showed a 1.2 order dependency in monomer concentration and that the mass transfer increased with the stirring rate. Nonideal dynamic responses were observed when changing the stirring rate. This was qualitatively described using a nonideal mixing model with a dead and active volume. It was found that introducing baffles and sparging considerably increased the mass-transfer ability at high stirring rates.