Theoretical synthesis in scientific disciplines is when new doctrines, the necessary revision of basic premises and the renewal of methodology are integrated in already existing theoretical knowledge. The need for theoretical synthesis is determined by changes in real practice, which require adequate scientific analysis. From this point of view, the paper analyses the political economy of socialism and Soviet sociology, which (along with scientific communism) were the major elements of Marxist social science in the USSR. Accordingly, the common methodological basis of both the political economy of socialism and Soviet sociology were the ideas of Marxism-Leninism and the model of socialist society which was based on them. It is shown that, despite repeated attempts undertaken by representatives of both disciplines, to critically analyse the concepts used in this methodology and to introduce new categories and conceptual additions, there was no significant update of the methodology. Therefore, necessary theoretical synthesis, in the course of which a phased “reset” and adaptation of theoretical knowledge to changing social reality takes place, did not occur either in the political economy of socialism or in Soviet sociology. The hypothesis that the “frozen methodology”of these sciences became the main obstacle to the adequate performance of their social functions, primarily prognostic and transformative, is justified. Thus, we associate the dissolution of the USSR with, among other things, the lag of the Soviet social sciences to reflect the needs of the socio-economic development of the nation. This lag is based on failures in the development and application of an adequate integral methodology for analysing the problems which built up in the USSR and to identify solutions.