The Belgian joined the Blues from Anderlecht for £18 million in 2011 but struggled to make an impact in the first team in his maiden year at the west London club.
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"I've had a great season and now I want to do better," he told reporters.
"After last year, a lot of people doubted me, but I kept believing in my qualities and worked hard.
"With the help of the fans and this club, I just couldn't fail here. From day one I have felt at home - like I was back in Brussels. That's why I have performed so well.
"They have helped me through the good and the bad days. I was really convinced the team would have a good season.
"It's been wonderful to be here."
The 20-year-old became only the third player, after Dirk Kuyt and David Bentley, to score a Premier League hat-trick against Manchester United, as the Baggies drew 5-5 with the Red Devils on the final day of the season.