Why do the pope and catholic church teach that islam is part of the plan of salvation?

No the Bible does not state Jesus is the only way to Heaven. It says those who Follow Christ will know the Kingdom of God/ will enter heaven/ shall not perish but have ever lasting life. It does not say however, those who do not believe in him don't have a chance of making it into Heaven. We, unlike the Protestants, believe one is saved by Grace. Not Faith alone. And the Muslims, do believe in God and if they live their life accordingly to be good people then God may give / guide them with his Grace. Along with the Jews.