mainland China vs Communist China

mainland China and Communist China both are nouns.

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

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

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

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

mainland China (noun)	Communist 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