A novel approach to determine the rate-limiting step in the Fischer-Tropsch (F-T) slurry process was developed using a simple computer model. This approach, unlike others, takes into account the water gas shift (WGS) reaction in the calculation of the importance of the gas-liquid mass transfer and makes use of the "singular kinetic path" concept proposed by Espinoza in 1993 which incorporates F-T kinetic expressions from different investigators. The predictions of the computer model showed that CO was the likely limiting reactant when using iron-based catalyst, since it is consumed by Fischer-Tropsch and water gas shift reactions and has lower mass transfer coefficients than those of H-2. It was also found that in the case of cobalt-based catalyst F-T synthesis is sensitive to the H-2/CO ratio of the inlet gas and H-2 was the likely limiting reactant, especially when the reactor was supplied with syngas having a H-2/CO ratio lower than the stoichiometric value. The Fischer-Tropsch synthesis in a slurry phase using the currently available iron-based catalysts and the given operating conditions could be considered a reaction kinetics-controlled process. The reactor performance could be improved by increasing the catalyst activity or catalyst concentration up to a maximum of 37-40 wt. %. In both cases, the reactor would approach a mass transfer-controlled regime, especially at high catalyst concentration which would increase gas bubbles coalescence tendency. The increase of the slurry reactor performance due to the increase of cobalt-based catalyst concentration appeared to strongly depend on the H-2/CO ratio in the syngas supplied to the reactor.