A mathematical model describing the vapor-phase synthesis of II-VI compound semiconductor nanoparticles in a counterflow jet reactor has been developed. The model was used to perform parametric studies in order to identify optimal operating conditions and reactor configurations that maximize particle yield, minimize polydispersity, and enable synthesis of particles with average size smaller than the quantum confinement threshold, i.e., quantum dots. The synthesis of zinc selenide nanoparticles by reacting vapors of dimethyl zinc with hydrogen selenide gas under isothermal conditions at 300 K was used as an example. In the experimental system, the two precursors were diluted in hydrogen carrier gas and each was fed into the reactor from a different inlet of art opposed-flow configuration. A steady-state reaction-transport model was developed to describe the laminar flow of the hydrogen carrier gas in the reactor, mass transfer of the precursors by convection and diffusion, and nucleation of particles through an irreversible reaction between the precursors. The reaction-transport model was coupled to an aerosol dynamics model describing particle transport by diffusion and convection, and particle growth by coagulation. The Galerkin finite element method was used to solve the resulting system of partial differential equations in the axial and radial directions of a cylindrical domain by assuming rotational symmetry. The only fitted parameter of the model was the activation energy for coagulation whose value was estimated by comparing model predictions with experimental data from a laboratory-scale reactor. A parametric study was performed to identify optimal operating conditions and a reactor configuration that maximizes particle yield, minimizes polydispersity, and enables the synthesis of particles with average size below the confinement threshold of 9 nm for zinc selenide.