We have investigated the dynamically asymmetric polymer blend composed of short (M-n approximate to 2 kg/mol) poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) chains focusing on the collective dynamics of the fast PEO component. Using neutron spin-echo (NSE) spectroscopy, the single chain dynamic structure factor of PEO was investigated and compared to results from molecular dynamics simulations. After a successful validation of the simulations, a thorough analysis of the RPA approximation reveals the composition of the experimentally measured total scattering signal S(Qt). Using the simulations, we show and calculate two contributions: (1) the relaxation of hydrogenated PEO against deuterated PLO, yielding the single chain dynamic structure factor of P:EO, and (2) the relaxation of the PEO component against the,PMMA matrix For the short chains presented here the second contribution shows a significant decay at higher temperatures While it was previously shown that, in the case of long chains, no relaxation is found. This difference is related to a decrease of the glass transition temperature which takes place with decreasing chain length. In a second step we analyze the approximations that are used when calculating the single chain dynamic structure factor using the Rouse model. For a system like pure PEO, where the dynamics follow the predicted Rouse behavior, excellent agreement is achieved. In the case of PEO in PMMA, however, the slow PMMA matrix strongly influences the PEO dynamics. As a result, the distribution functions show a strong non-Gaussianity, and the calculation of S(Qt) using the Rouse approximation fails even considering nonexponential Rouse mode correlators.