The nascent internal state distribution of NO( X (2)Pi) generated from the reaction, S(D-1) +N2O-->NO+NS, has been determined by utilizing the laser-induced fluorescence (LIF) technique. The average vibrational energy of NO relative to the statistically expected value is found to be 37%. This amount is obviously smaller than that of the fragment N O-16 of the isovalent reaction O-18(D-1)+N-2 O-16-->N O-18+N O-16, though it is still larger than that of (OH)-O-18 produced from the O-16(D-1)+H-2 O-18 reaction. To interpret the observed difference in the product energy partitioning, we have applied the quantal intramolecular vibrational-energy redistribution (IVR) representation to the energy mixing in the collision complex. Using a local-mode vibration model with momentum couplings, we have extracted the crucial factors determining the energy partitioning in these reactions. The reaction system consisting of only heavy mass atoms generally has a large vibrational coupling and a large density of states, both of which favor the rapid energy mixing during the short-lifetime of the intermediate complex.