Communist China vs mainland China

Communist China and mainland China both are nouns.

Word	Noun	Adjective	Verb	Adverb
Communist China	Yes	No	No	No
mainland China	Yes	No	No	No

As nouns, Communist China and mainland China are synonyms defined as:

Communist China and mainland China: a communist nation that covers a vast territory in eastern Asia; the most populous country in the world

Other synonyms of Communist China include Cathay, China, People's Republic of China, PRC, Red China.

Communist China (noun)	mainland China (noun)
a communist nation that covers a vast territory in eastern Asia; the most populous country in the world	a communist nation that covers a vast territory in eastern Asia; the most populous country in the world