Molecular dynamics simulations were performed to study systematically the rapid mixing of a polymer blend in solution with a miscible nonsolvent. In agreement with experiments, we observe that polymers self-assemble into complex nanoparticles, such as Janus and core shell particles, when the good solvent is displaced by the poor solvent. The emerging structures can be predicted on the basis of the surface tensions between the polymers as well as between the polymers and the surrounding liquid. Furthermore, the size of the nanoparticles can be independently tuned through the mixing rate and the polymer concentration in the feed stream; meanwhile, the composition of the nanoparticles can be controlled by the polymer feed ratio. Our results demonstrate that this process is highly promising for the production of structured nanoparticles in a continuous and scalable way with independent and precise control over particle size, morphology, and composition. Such tailored nanoparticles are highly sought after in various scientific and industrial applications, and our theoretical findings provide important guidelines for designing appropriate experimental fabrication processes.