you people are confusing communism, the Marxist concept of the "whithering away of the state" with Communism the 20th c political system. the former, whether it is possible or not, has no place in civ, since it is not part of history, but comes at the end of history, when the class strugle has ceased.
Communism as a practical phenomenom is a POLITICAL system - it means rule by a centralized proletarian political party following Marxist principles.
While one would expect such a regime to implement a socialist economic system, it is capable of implementing an alternative one for pragmatic reasons, as the USSR did under the NEP. Whether a party that implements a capitalist economy for the long term (IE current China) can still be called a Marxist party is questionable.
Of course a socialist economy can be implemented without a communist regime - it can be implemented by a military despotism, and in theory at least, by a democracy (though there are real conflicts between the state power availble under an extreme form of socialism and the limits on state power requried for a democracy to function)
Stalinism involves the emergence of a single despot over and above the proletarian party.
"Eurocommunism" posited the emergence to power of a communist (centralized proletarian) party to power via democratic means, and maintianing democratic forms. Whether this program could ever have been accomplished (or was even sincere) is open to question. It should not be confused with the older democratic socialist movement.
Socialism should be kept distinct from "statism" at least conceptually, though not necessarily in Civ. Socialism involves the intervention of the state in the economy on behalf of the working class - statism can involve the intervention of the state in the economy for other reasons - Brazil under the military is perhaps the clearest example. This would seem to point to 2 seperate social engineering factors - one for market economy versus central planning, and one for social justice, which would allow for a central planned economy without pusuit of social "justice".
Note well that socialism is often called "statism" by its enemies, who are skeptical of its motivations.