In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: CONCLUSIONS : Methadone is associated with QT prolongation and higher reporting of syncope in a population of heroin addicts .
### Output Text:
CONCLUSIONS : <span class="chemical">Methadone</span> is associated with QT prolongation and higher reporting of syncope in a population of <span class="chemical">heroin</span> addicts . 