Production of newsprint has become Canada’s largest industry after agriculture, new mills are being built in several provinces.

I want to turn the above sentence to a nonrunning sentence. For my headline project, its my first sentence, but that comma doesnt belong there, a semicolon does, but a semicolon just ruins the whole sentence, cause I don't want there to be a semicolon, what to add?

I like it better as two sentences, but there is nothing wrong with using a semicolon.

Inserting the word "because" at the very beginning and "paper" before "mills" will do the job with one sentence.

To turn the sentence into a nonrunning sentence without using a semicolon, you can add a conjunction. Here's a revised version of the sentence:

"Production of newsprint has become Canada’s largest industry after agriculture, and new mills are being built in several provinces."

In this version, the conjunction "and" is added to join the two independent clauses and create a grammatically correct sentence.