OK, if you're looking to use the gerund in your sentence then maybe something like : 'Before the film begins, the ushers will have been collecting the tickets' ? I would think you need an 'active' form as opposed to a 'passive' form to be able to use the progressive tense, so you need to change the subject, which can no longer remain 'the tickets' but has to be the person that collects them. How does it sound to you ?
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