The theory of flow-induced coalescence, based on the switch between the rates of approach of spherical and flattened droplets, is generalized to systems with different droplet radii. Probability, P-c, that the droplet collision will be followed by their fusion is calculated for systems containing droplets with two different droplet radii as a function of the average droplet radius. Analysis of the results obtained shows that P-c is a gradually decreasing function of the average droplet radius, < R > in broad range of < R > for systems with a large width of droplet radii distribution. The dependence of P-c for a polydisperse system should be considered also if experimentally determined average particle size is regarded as a result of the competition of the droplet breakup and coalescence in flowing polymer blends.