A numerical approach to the determination of gas kinetics in the case of non-planar nanostructure growth via the selective-area metal-organic chemical vapor deposition is developed. The direct simulation Monte-Carlo method is utilized to model the rarefied gas flow of precursor particles nearby the substrate. The computation is performed for the GaAs nanowire growth via the selective-area metal-organic chemical vapor deposition. The model allows the quantitative description of the decrease of nanowire length with the increase of distance between nanowires (the so-called synergetic effect). The optimal pitch of the mask that corresponds to the maximal nanowire length is found for typical growth conditions. In particular, our calculation shows that the optimal pitch increases with the increase of the nanowire diameter.