Traditional methods for system-level design of Delta---Sigma (źΣ) modulators typically assume linear modeling of the modulator, in which quantization noise is modeled as additive independent white noise. But it is well known that the źΣ modulator is a non-linear system and linear modeling is only an approximation. Also, circuit-level non-idealities, which may greatly change the modulator behavior, are typically neglected at system-level design. As a result, system-level modulator designs obtained from traditional methods may not be realistically optimal. This paper presents a system-level post-optimization method for źΣ modulators so that modulator designs initially obtained from traditional methods are post-optimized considering non-linear and non-ideal characteristics of źΣ modulators including both quantization noise and circuit-level non-idealities. The post-optimization algorithm is based on Finite Difference Stochastic Approximation due to the stochastic nature of modeling of some circuit-level non-idealities in system-level design. During the post-optimization run, each candidate design is simulated for performance measures and stability has always been a must constraint. Results on two źΣ modulators have shown that post-optimized modulator designs outperform the original designs from traditional methods.