Here's why:
* Noun clauses act as nouns. They can be the subject, object, or complement of a sentence.
* In this sentence, "what my great dream is" is the predicate nominative, which means it renames the subject "My great dream".
However, the provided sentence is incomplete. It needs a word or phrase to complete the thought. For example:
* My great dream is to travel the world.
* My great dream is to become a doctor.
In these complete sentences, "to travel the world" and "to become a doctor" are the noun clauses that function as predicate nominatives.