From the
http://lotrtcgwiki.com/rules/crd.pdf - "playing a card" entry, point 5:
Respond to the playing of The Card (and to losing initiative if necessary). Responses or
triggered actions that respond to the playing of
The Card happen now. If The Card has game text on it that triggers "When you play..." The Card, it happens now. Other cards may respond to the card being played as well. These are handled in the manner described under actions and action procedure. If The Card was a Free People's card and it leaving your hand causes you to lose initiative, each player may respond to you losing initiative now.
The relevant part is in bold. "Each time a Shadow condition is played, you may exert a minion.", so this triggers and can be used ONLY during the step 5 of playing the Shadow Condition.
To add on top of that, bearing in mind that both playing Shadow Condition and Minion are Phase actions for Shadow phase, from Comprehensive rules:
"Each phase action must be completely performed before another phase action can be performed. Phase actions cannot be combined."
Therefore when you play Shadow Condition, the "Each time a Shadow condition is played..." can be used during step 5. Only after you get through the whole process of "playing a card" (steps 1 through 7), you can start playing Minion, hence at that time it's impossible to use the "Each time a Shadow condition is played..."
Hope that helps.
