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There's no need for adding that second summer because you only add it to make analysis easier (linear) for you as a designer. Matlab has no problem analyzing non linear systems. You should add a comparator/quantizer, this will be useful for comparing your analysis to the results given by Matlab
Also why did you insert a quantizer before the integrator? That way no noise shaping will ever take place.
Bottom line : remove the 2nd summer and the white noise blocks, remove the quantizer placed before the integrator, and place a quantizer after the integrator and you got yourself a delta sigma modulator.
This link has video lectures concerning Delta Sigma Modulators, you'll also find some simulink simulations in the slides