This sentence contains a compound predicate ...right?
She Placed the shells in a plastice bag while he dried himself off.
Here's a sentence with a compound predicate:
She placed the shells in a plastic bag and then dried off the sand toys.
To have a compound predicate, you must have two or more main verbs (and any other words that go with them).
The sentence you wrote is a complex sentence with one main clause (she placed the shells in a plastic bag) and one subordinate clause (while he dried himself off). Each clause has its own subject and verb.