  The multinucleon transfer reactions in collisions of $^{136}$Xe+$^{198}$Pt at
incident energies $E_{\textrm{lab}}=$5.25, 6.20, 7.98, 10.0, and 15.0
MeV/nucleon are investigated by using the improved quantum molecular dynamics
model. It is found that 6.20 MeV/nucleon is the optimal incident energy for
producing the neutron-rich heavy nuclei. About 80 unknown neutron-rich nuclei
might be produced in this reaction with cross sections from 10$^{-6}$ to
10$^{-2}$ mb. The angular distributions of the neutron-rich isotopes are
predicted.
