In Critique of the Gotha Programme:

What we have to deal with here is a communist society, not as it has developed on its own foundations, but, on the contrary, just as it emerges from capitalist society; which is thus in every respect, economically, morally and intellectually, still stamped with the birth marks of the old society from whose womb it emerges. Accordingly, the individual producer receives back from society – after the deductions have been made – exactly what he gives to it. What he has given to it is his individual quantum of labour. For example, the social working day consists of the sum of the individual hours of work; the individual labour time of the individual producer is the part of the social working day contributed by him, his share in it. He receives a certificate from society that he has furnished such and such an amount of labour (after deducting his labour for the common funds), and with this certificate he draws from the social stock of means of consumption as much as the same amount of labour costs. The same amount of labour which he has given to society in one form he receives back in another.

Such a system requires centralized planning, Marx’s entire reason for predicting Socialism to overtake Capitalism came from Marx’s analysis of Capitalism’s centralizing factor. As industry gets more complex, it grows, until everything is owned in common after revolution and gradual expropriation from Capitalists.

True, but we know that one. He also never had a plan to achieve communism either. The devil’s in the details of HOW we get there.