Abstract: Disclosed are methods and systems for producing polymer-DNA nanoparticles of a predetermined size. In one example, a method includes mixing together a first solution comprising deoxyribonucleic acid (DNA) with a second solution comprising a cationic polymer to obtain a polyplex solution, and at a predetermined time subsequent to mixing together the first solution and the second solution, adding a polyplex stabilizing agent to stabilize the size of the polyplex. In this way, transfection efficacy of the polymer-DNA nanoparticles may be improved, in particular with reference to transfection of suspension cells for production of viral vectors.